For example, at a gas station or the like, gasoline, light oil or kerosene is stored in a liquid storage tank that is installed underground. Under the Fire Service Law, inspection for presence or absence of leakage from this liquid storage tank is obligatory immediately after the installation and periodically. Inspection methods typically include a pressurizing test method, a micro-pressurizing test method, and a micro-depressurizing test method.
The pressurizing method includes a gas pressurizing method and a liquid pressurizing method. In both cases, gas or liquid is encapsulated in a tank as an inspection target, and after the inside of the tank is pressurized to reach a prescribed pressure and left in a still state, determination is made on the basis of whether the pressure drop value maintains a value within a reference range. Nitrogen gas (N2) is used in the gas pressurizing method, and water is mainly used in the liquid pressurizing method.
In the micro-pressurizing method, nitrogen gas is encapsulated in the tank, and after the inside of the tank is pressurized to reach a prescribed pressure and left in a still state, determination is made on the basis of whether the pressure drop value maintains a value within a reference range, as disclosed in Patent Reference 1. In this case, since it is aimed at inspecting only the inner wall of the gaseous phase part of the tank, a lower pressure than that applied in the pressurizing method is applied.
In the micro-depressurizing method, the gaseous phase part of the tank is depressurized and it is determined whether the pressure rise value is kept at a reference value. Also in this case, since depressurization is carried out in the gaseous phase part, the inspection target is the inner wall of the gaseous phase part only.
Patent Reference 1: JP-A-10-19717
Patent Reference 2: U.S. Pat. No. 5,319,956